Known mechanical systems for dry transportation and/or cooling of loose material—for instance ash or waste that have undergone a combustion process—provide various solutions for containment, conceived to confine the material in a transport region, preventing its outlet to the external environment or anyhow its pouring out of said confined region. A system for extraction and transport of the aforedescribed type is that of EP 2 480 830.
In such mechanical transport systems, particularly those normally employed for ashes produced in a combustion chamber of an incinerator, it is necessary to ensure operation continuity of the system and therefore of the plant in which it is included. Crucial to such reliability of operation is the avoiding of interference of heterogeneous materials, above all metal ones, with stationary and moving parts of the extraction and transport system.
In particular, the mechanical dry transport systems considered herein are based on a metal conveyor belt enclosed in a casing and associated with appropriate means for collecting fines from the bottom of the casing. Such collecting means is comprised, e.g., of chains or pendulum or shovel systems hinged to the belt.
Known belt conveyors do not guarantee a high reliability of operation when the material to be transported is of heterogeneous type, and in particular comprised of material both fine and large-sized, of varying chemico-physical nature and, above all, in the presence of incompressible materials, like e.g. metal matters. These materials can come out, or anyway interfere with the mechanical solutions for containment, or even fall onto the bottom of the casing, thereby acting on the fines recovering means. In that case, loose material fallen onto the bottom can block recovering means operation, or anyway cause bindings and stoppages of the main mechanical transport system, compromising its reliability and causing unforeseen and potentially lengthy downtimes.
The abovementioned belt conveyors are typically applied not only for the extraction of material from process boilers or furnaces, but also for transport, post-combustion, cooling or drying operations carried out with a gas that is inlet into the belt conveyor environment, typically in countercurrent to the material transported. In particular, there may be provided the inletting of a hot gas, or of a gas for chemical or physical treatments deriving from gas interaction with the material transported, or also of ambient air for cooling.
In presence of gas for the abovementioned treatments, in particular thermal treatments or of other nature, it may be necessary both to permeate the material with such gas during transport on the belt, and to maintain the degree of reduction reached by the material in upstream process furnaces (e.g. by Nitrogen). In that case, treatment gas should not be dispersed in the environment, both to avoid consumption thereof and to safeguard the environment.
In particular, when the material to be transported is heavy ash, coming from a boiler, which ash falls into the belt system from the bottom of said boiler, known dry transport systems provide for air-assisted cooling of the ash bed present on the belt. Cooling air can be recalled into the transport system by the negative pressure present in the boiler, by passing through specially provided openings of the containment casing of the belt conveyor. Air therefore crosses the system and the ash bed in countercurrent to the direction of advancement, thereby operating ash and apparatuses cooling. An extraction and cooling system of the aforedescribed type is that of EP 0 252 967.
In known systems of the aforedescribed type, ash cooling efficiency depends upon the exposed surface available for heat exchange with air, and on the ability to attain an effective conveying of the air at the ash, allowing the desired interaction level.
Moreover, cooling air is returned into the boiler from the bottom thereof, bringing in the heat content removed from the ash. However, the amount of cooling air that can re-enter the boiler without interfering with combustion process efficiency is limited and ranging from 1.0 to 1.5% of total combustion air.
Hence, a further perfectible aspect of the known transport and cooling systems is linked to heat exchange modes between transported material and cooling air. Such perfectibility is particularly important in case of high flow rates of produced ash.
On the basis of what has been explained hereto, for the transport, the post-combustion and/or the dry cooling of heterogeneous material, e.g. coming from incinerators but also from solid-fuel boilers, and for the transport in inert environment in case of material that has undergone a reduction treatment, it is necessary to deal with the need for mechanical transport continuity and reliability, as well as with needs for optimal cooling without interfering with in-boiler combustion process efficiency, as well as again for needs of treatment gas containment in the transport zone.